don't reset rate when changing video

Set the defaultPlaybackRate property also on the video element.
This prevents the playback rate resetting to normal again if the video element's src attribute is changed after the plugin had been toggled.

This is useful for jPlayer when in playlist mode to allow the video's speed to persist when the next playlist item is selected and avoids needing to open the addon menu and select the speed again for every video.

Only persists for the single video element instance and not page refreshes, leaving the page or when the video element is destroyed and re-created.